Certain types of fire detecting apparatus utilize an optical detector comprising a pair of photo-cells connected in series across a voltage source. The cells are responsive to different wave lengths of light, so that on receiving sunlight or incandescent light, both cells respond by a drop in resistance, so that the voltage at the junction between the cells changes only slightly. However, when exposed to light with substantially only infra-red or red components, the resistance of one cell drops substantially and the resistance of the other cell drops little, if at all. Hence, assuming that the positive pole of the power source is connected to the red-responsive cell, the voltage at the junction rises appreciably, and the rise in voltage is used as the input to the amplifier to activate an alarm. A detector of this type is shown in U.S. Pat. No. 3,188,593 issued June 8, 1965.
One problem associated with detector systems of this type is the testing of the operability of the detector system. In the case of aircraft installations, such testing should be done in preparing the aircraft for flight. Although during overhaul procedures, the fire detection system may be tested by shining a red light on the detector cells, such a procedure is obviously impractical for a daily pre-flight check, and it is therefore necessary that some simple, rapid test procedure be available in the cockpit, so that the fire detector system can be tested by the pilot before each take-off, or, if desired, in flight.